1. Field of the Invention
This invention relates to a slip construction for supporting tubular members such as well casings, tubular hangers packer inserts, tubing slips, liner hangers, and the like, and more particularly to a well casing slip having a slip insert construction that provides reduced surface deformation of a well casing while maintaining sufficient gripping force thereon to hold and support such a casing in the course of well drilling operations.
2. Description of the Prior Art
In the course of drilling oil and gas wells, a bore hole is drilled in the earth to a sufficient depth to provide communication with and to permit withdrawal of the subterranean oil and gas deposits. A well casing of generally tubular configuration, such as a pipe, is inserted into the bore hole in order to provide an escape path for drilling debris, for the oil or gas, and simultaneously to prevent the collapse of the side walls of the bore hole and the consequent filling or plugging up of the hole with earth. The casing is handled by means of a device referred to as an elevator, which includes a generally bowl-shaped annular housing within which several wedges, commonly referred to as slips, are positioned, the slips having inwardly directed gripping surfaces to engage and grip the exterior surface of a casing.
The gripping surfaces include numerous inwardly directed teeth that are adapted to engage the outer surface of the casing in order to securely hold it in position and permit it to be transported vertically into the bore hole. Generally, the slip insert materials in common use are very rigid and hard materials that have a damaging effect on the pipe they are intended to hold. The damaging effect is manifested in partial embedment of the insert teeth in the surface of the wall of the casing. The embedment results from the gripping forces and from the hardness of the slip assembly (slip and insert) material, which is generally two to three times higher than the surface hardness of the usual oil field tubular goods. Additionally, the hardness of the slip insert material increases with use because of the effect of work-hardening. The embedment of the teeth in the casing surface causes depressions, resulting in localized higher stresses in the casing when it is subjected to axial loading, because the portions of the casing having the depressions have a smaller effective cross-sectional area. Additionally, some of the slips in general use are retained in an annular retainer commonly referred to as a bowl, and any slight misalignment of the innermost surfaces of the slips with respect to the casing surface results in unequal hoop stresses being applied to the casing, further contributing toward localized surface deformation, bending of the casing wall, and consequent increased localized stresses at those areas.
In addition to well casings, other tubular members such as tubing hangers, liner hangers, packer inserts, and the like, also involve gripping operations, either externally or internally of the tubular member, and similar problems are involved in the gripping and handling of such members.
It is thus an object of the present invention to overcome the above-noted problems.